Shadows bleeding are ALWAYS due to modelling errors:
-coplanar faces : causes surface shimmering and in some occasions shadow leaks
- Internal vertical faces: when using the extrusion sub-modelling technique you always have the original face left. If you don't delete it, you're 100*% sure to get bleeding.
- flipped faces
- non closed surfaces at the top of the model: the shadow projection goes top bottom. If for some reason there is no top face, but only a bottom one, you're probably going to have shadow problems.
It can take some time to find the offending polygons or gaps, but it is always possible to solve the problem...
